Problem: A polynomial of degree $13$ is divided by $d(x)$ to give a quotient of degree $7$ and a remainder of $3x^3+4x^2-x+12$. What is $\deg d$?
Let $f(x)$ be the polynomial of degree $13$ and let $q(x)$ be the quotient when $f(x)$ is divided by $d(x)$. Let $r(x) = 3x^3+4x^2-x+12$. Then we have
$$f(x) = d(x)\cdot q(x) + r(x).$$where $\deg q = 7$.

Since $\deg r = 3$, we need to have $\deg(d\cdot q) = \deg f$ which means $\deg d + \deg q = \deg f$.  So $\deg d = 13-7 = \boxed{6}.$